Elimination of pathogen-infected cells requires their sequential recognition by cells in the innate and acquired immune systems. A unique category of T cells collectively referred to as 'innate T cells' kinetically bridge the innate and acquired immune responses. Interestingly, the activation of innate T cells can be restricted by the expression of MHC-like molecules or class Ib molecules. The ability of innate T cells to respond rapidly stems from their lack of dependence on clonal expansion, resulting from less discriminating receptor-ligand interactions. By comparison, classical MHC molecules restrict conventional CD4 and CD8 T cells in the acquired immune response, and they require 10-14 days to clonally expand to sufficient numbers due to highly specific receptor-ligand interactions. The experiments in this grant will characterize the novel class Ib molecule MR1 that is evolutionarily conserved in mammals and restricts the development of mucosal-associated invariant T cells or MAIT cells in a manner dependent upon the commensal flora and B cells. MAIT cells have several features consistent with their function as innate T cells including i) invariant T cell receptor alpha chains, ii) the ability to rapidly release cytokines, and iii) apparent recognition of an evolutionarily conserved ligand. However, the nature of the MR1 ligand is unknown as is the physiological role of MAIT cells. In this application we will test whether MAIT cells detect Mycobacterium tuberculosis-infected cells and are critical components of host resistance to this pathogen. In addition, we will elucidate the nature of the MR1 ligand, the pathway by which it is presented, and the crystal structure of a ligand/MR1 complex. Cellular, molecular and structural dissection of MR1 antigen presentation to MAIT cells will critically define the mechanism and importance of this pathway in protective immunity to bacteria.